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Many plants can cause either irritant or allergic skin rashes. The single most common cause of allergic skin reactions in the United States is contact with poisonous plants such as poison ivy, poison oak or poison sumac, according to the American Academy of Dermatology. Common house and garden plants (including daffodil, chrysanthemum, sunflower and dandelion) can also cause allergic skin rashes in sensitive people.

The parts of the plant that cause the allergic reaction or irritation vary and include hairs, sap, oils, leaves, stems, petals, roots and bulbs. Outdoor plants, both weeds and native varieties, may cause allergic and irritant skin rashes. Plants with thorns, such as stinging nettles and thistles, are well known for irritating the skin.

Rashes caused by an allergic reaction to plant parts are called allergic contact dermatitis. Rashes caused by irritation due to plants are called irritant contact dermatitis. Plant dermatitis can result from direct contact with a plant, indirect contact with a plant (such as touching something that touched the plant) or contact with airborne plant particles that are either inhaled or come into contact with skin. Some plants (e.g., limes, wild parsnip) only cause a reaction when sunlight is present on the skin at the moment of contact. In such instances, touching the plant produces a chemical reaction between plant compounds and sunlight (phytophotodermatitis).

Any plant may cause an allergic reaction in a sensitive person, but the most common sources of plant rashes are poison ivy, poison oak and poison sumac. According to the American Academy of Dermatologists (AAD) 10 million to 50 million Americans develop an allergic rash after contact with these poisonous plants. Characteristics of these plants include:

• Poison ivy. A weed with three shiny green leaves and a red stem, it typically grows in vines and can be found along riverbanks. Resins of these plants cause a reaction in many people when their skin comes into contact with the plant. In addition, smoke from burning poison ivy plants can cause the same type of skin rash in people exposed to the smoke. Poison ivy is found throughout the United States except in the Southwest, Alaska and Hawaii. In some areas – such as in the far northern and western United States, the areas around the Great Lakes, and Canada – poison ivy grows as a shrub.
  
  
• Poison oak. Has the same three-leaf structure as poison ivy, but grows in the form of a shrub in the eastern United States and a shrub or a vine in the western United States. It is found primarily on the West Coast.
  
  
• Poison sumac. Grows as a woody shrub, with each stem containing seven to 13 leaves arranged in pairs. It grows in standing water in the peat bogs of the Northeast and Midwest and in swampy areas of the Southeast. It grows most abundantly along the Mississippi River, but can also be found less commonly in other regions.

All three of these plants irritate the skin through their resin, an oily sap known as urushiol that sticks to just about everything it comes into contact with. Urushiol is a nearly invisible colorless or pale yellow oil that oozes from cut or crushed roots, stems and leaves.  The rash itself is not contagious, although there is a slight chance that a person can be affected after coming into direct contact with resin that remains on another person’s skin or on clothing, pets or other objects.

Plants that are part of the same family (based on common features and similarities in structure) can create similar allergic reactions. For instance, onions are in the same family as lilies because of the type of root they share (a bulb) and their similar leaf structure. Yet onions have unique flavor characteristics that other plants in the lily family do not have. Nevertheless, someone who is allergic to lilies is advised to avoid onions, garlic, hyacinth, tulips and other plants in the same family. Lily sap can trigger an allergic skin rash and the rest of this plant family may cause an allergic rash in people with lily allergies.

Families of plants will include a wide variety of plants that do not look or taste similar (on casual examination) but share important characteristics that make them scientifically comparable. For example, a person who is sensitive to poison ivy, oak or sumac may also have reactions to latex (rubber) and to mango, cashew, celery, carrots and fig plants. This is because they are all from the same plant family.

The sensitivity people have to poisonous or irritating plants varies from person to person. A person's sensitivity to plants also tends to decline as they age.